The problem of determining the quality of coil insulation in high voltage electric machines has been in existence for a long time. Generally, the causes of deterioration of the coil insulation cannot be attributed to a single condition. In practice, it has thus been necessary to follow an expensive procedure of rewinding machines otherwise in perfect condition, since no methods were reliable to ensure detection of an imminent penetration of the insulation of the machine.
Due to an improved understanding of the deterioration phenomenon, it has been recognized that the insufficient strength of the insulation of apparently sound coils is caused by the generation of localized partial discharges inside the insulation, which appear above a certain voltage threshold. However, this information did not solve the problem, because the localized partial discharges inside the insulation escape direct measurement.
According to the generally accepted conventional view, the partial discharges localized inside the insulation take place at gaseous occlusions distributed in vacuoles more or less over all of the high voltage insulation. At certain voltages, partial discharges are generated inside a rising number of vacuoles, or cavities. The capacity of the coils varies as a function of the applied voltage, as the inside of these vacuoles, becomes conducting. In view of these findings, for example Terase (Electr. Eng. Jap., February 1973) has proposed a quality criterion based on the variation of the capacity curve of coils as a function of the applied voltage. This characteristic may be determined by means of a capacity measuring bridge, for example a Schering bridge. The Terase method in certain cases is capable of yielding valuable information. In numerous other cases, however, the precision with which the characteristics essential for interpretation of the data is inadequate. In still other cases, the effects investigated are completely masked by other phenomena.